Hagel, a former Republican senator, has served as Pentagon chief since early 2013. Obama is expected to announce the resignation Monday.

The official insisted on anonymity because this person was not authorized to discuss the matter publicly ahead of Obama's official announcement.

A senior defense official said that Hagel submitted his resignation letter to Obama on Monday morning and the president accepted it. Hagel agreed to remain in office until his successor is confirmed by the Senate, the official said.

The official says that both Habel and Obama "determined that it was time for new leadership in the Pentagon." The official adds that they had been discussing the matter over a period of several weeks.